With the advance of science and technology, a copier can produce high-quality replicas by using an equipment of photocopy, e.g. printer or scanner, which already can precisely copy documents.
Recently, issues of copyright and brand protection are getting more attention gradually after documents and books are copied illegally and frequently from time to time. The anti-counterfeiting technologies used on documents are produced continuingly, for instance, the watermark, the gravure, the infrared ink, micro prints, windowed security thread and so on. However, the above-mentioned anti-counterfeiting technologies need special equipments and/or materials, and thus are costly. As a result, it will cause limitation to widely use these anti-counterfeiting technologies for protecting common publication of books and security documents.
In these anti-counterfeiting technologies, a digital watermark with hybrid halftone dots can provide the anti-counterfeiting capability without using the special equipment and/or materials. The technology mixes the amplitude modulation (AM) halftone dots with the frequency modulation (FM) halftone dots to produce a shadow pattern of watermark, wherein matching parameters therebetween are found by making a density-calibration chart (i.e. density matching table) to adjust the distribution density of the AM halftone dots and the FM halftone dots, and the watermark is finally printed on the paper. The watermark can appear to provide the anti-counterfeiting effect after the paper is copied or scanned.
For example, referring now to FIGS. 1 and 2, a watermark 1 providing hybrid halftone dots is illustrated. As shown, the watermark 1 comprises a plurality of AM halftone dots 11 and FM halftone dots 12, wherein the AM halftone dots 11 and FM halftone dots 12 provide different dot gains due to varied microstructure of halftone dots. The hidden effect of the watermark 1 is thus affected by the different dot gains. To ensure the hidden quality of the watermark 1, the distribution density of the AM halftone dots and the FM halftone dots must be adjusted according to density-calibration chart when mixing the AM halftone dots with the FM halftone dots. Otherwise, the distribution density of the halftone dots is unstable in the process because the dot gain of the AM halftone dots and the FM halftone dots are inconsistent. Thus, it needs careful adjustment to control the hidden quality of the watermark 1.